1. Field of the Invention
The present invention relates to a method for producing an L-amino acid using a microorganism. L-amino acids are used in various fields such as for use in seasonings, food additives, feed additives, chemicals, and drugs.
2. Brief Description of the Related Art
L-amino acids such as L-threonine and L-lysine are industrially produced by fermentation using amino acid-producing bacteria such as Escherichia bacteria. As these amino acid-producing bacteria, bacterial strains isolated from nature, artificial mutants of those bacterial strains, recombinants of those bacterial strains in which L-amino acid biosynthetic enzymes are enhanced by gene recombination, or the like, are used. Examples of the methods for producing L-threonine include, for example, the methods described in Japanese Patent Laid-open (Kokai) No. 5-304969, International Publication WO98/04715, Japanese Patent Laid-open No. 05-227977, and U.S. Patent Published Application No. 2002/0110876. Examples of the methods for producing L-lysine include, for example, the methods described in Japanese Patent Laid-open No. 10-165180, Japanese Patent Laid-open No. 11-192088, Japanese Patent Laid-open No. 2000-253879, and Japanese Patent Laid-open No. 2001-057896.
In the industrial production of L-amino acids by fermentation, saccharides, i.e., glucose, fructose, sucrose, blackstrap molasses, starch hydrolysate, and so forth, are used as carbon sources. Frequently used in methods for producing an L-amino acid by fermentation as a carbon source are saccharification products of starches derived from higher plants such as corn and cassava. These have low moisture content and high starch content, and therefore it is easy to industrially obtain starches from them. On the other hand, although starches contained in microalgae are present at an amount per dry weight unit comparable to that of corn or cassava, the dry weight of the algae per weight unit of culture medium does not reach 1%. The process of separating alga bodies, dehydrating them, disrupting the cells, extracting starches, and purifying the starches is complicated and difficult. Although ethanol fermentation using starches of microalgae is described in U.S. Patent Published Application No. 2006/135308, U.S. Patent Published Application No. 2007/0202582, and Matsumoto, M. et al., 2003, Appl. Biochem. Biotechnol., 105-108:247-254, the results of the ethanol fermentation are not described. Further, any example of use of saccharified starches of microalgae for amino acid production has not been shown so far.
It is known that Escherichia coli, which is a typical amino acid-producing bacterium, can grow using glycerol as a sole carbon source (Lin, E. C. C., 1996, pp. 307-342, In F. D. Neidhardt (ed.), Escherichia coli and Salmonella Cellular and Molecular Biology/Second Edition, American Society for Microbiology Press, Washington, D.C.), and can grow using long chain fatty acids having 12 or more carbon atoms as the sole carbon source (Clark, D. P. and Cronan Jr., J. E., 1996, pp. 343-357, In F. D. Neidhardt (ed.), Escherichia coli and Salmonella Cellular and Molecular Biology/Second Edition, American Society for Microbiology Press, Washington, D.C.). Therefore, it is described in Brenner, D. J. and Farmer III J. J. (Family I., 2005, pp. 587-669, In: D. J. Brenner, N. R. Krieg and J. T. Staley, Editors, Bergey's Manual of Systematic Bacteriology, Volume Two: The Proteobacteria Part B: The Gammaproteobacteria, Springer, New York), that Escherichia coli can assimilate both long chain fatty acids and glycerol, which are the hydrolysis products of fats and oils, but Escherichia coli does not have lipase activity, and therefore it cannot directly assimilate fats and oils. Furthermore, it is also known that solubility of long chain fatty acids is generally extremely low, and the measurement results of the solubility are described in Vorum, H. et al., 1992, Biochimica et Biophysica Acta, 1126:135-142, i.e., although solubility of lauric acid is not lower than 0.1 g/L or more, solubility of oleic acid is not higher than 0.0003 g/L, and that of palmitic acid is not higher than 0.00000003 g/L. Therefore, it is difficult to simultaneously assimilate highly water-soluble glycerol and fatty acids, and there has not been reported to date L-amino acid production based on direct fermentation utilizing hydrolysates of fats and oils, which is a mixture of long chain fatty acids and glycerol, as a carbon source.
As for soybean and Elaeis guineensis (oil palm), which are oil plants generally used for production of edible oil, beans or fruits thereof contain about 20% of fats and oils. As for microalgae, there are known microalgae producing fats or oils, and the yield of fats and oils per area much exceeds that obtainable with the oil plants as reported in Chisti Y., 2007, Biotechnol. Adv., 25:294-306. However, the process of separating algae, dehydrating them, disrupting the cells, extracting fats and oils and purifying them is complicated and difficult, as in the case of starches. Therefore, there have so far been no reports about L-amino acid production based on direct fermentation utilizing fats and oils originating in algae.
Further, although methods for extracting organic substances derived from chlorella have been reported (Japanese Patent Laid-open No. 9-75094, International Publication WO2006/095964, and U.S. Patent Published Application No. 2007/0202582), it has been considered that disruption is preferably performed by a high temperature reaction. Moreover, there has so far been no report about production of L-amino acids by direct fermentation utilizing a processed product obtained by the aforementioned methods as a carbon source. Furthermore, it has also been known that nucleic acid-related compounds can be increased by autolysis of chlorella (Japanese Patent Laid-open No. 62-278977), but there has so far been no report about production of L-amino acids by direct fermentation utilizing a processed product obtained by such a method as mentioned above as a carbon source.